1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to adaptively filtering an input image in color domain units in which the input image is divided into a plurality of color domains, a plurality of filter coefficients are respectively calculated for the color domains, and the input image is adaptively filtered in units of the color domains according to the properties of the color domains.
2. Description of the Related Art
In order to meet various user demands, the functions of digital imaging devices such as monitors, scanners, and printers have diversified and improved. Digital imaging devices may adopt different color spaces or color models according to their purpose of use. Color models are classified into device-dependent color models and device-independent color models. Examples of the device-dependent color models include red-green-blue (RGB) models and cyan-magenta-yellow-black (CMYK) color models which are a type of subtraction color space models. Examples of the device-independent color models include CIE color models such as a CIEL*a*b* model, a CIE XYZ model, and a CIE LUV model. In the meantime, CMYK color spaces are widely used in the field of printing, whereas RGB color space are widely used in the field of computer graphics based on computer monitors, e.g., the field of web graphics.
Some colors in digital images printed by printers or displayed by display devices can be easily registered in a person's memory and can be remembered for a long time, and these colors are referred to as memory colors. Memory colors are colors of objects which are familiar to most people, and thus, most people automatically perceive these colors as they are without even looking at the objects. The performance of printers or display devices is affected by how properly the printers or display devices can represent memory colors. Therefore, a variety of filtering techniques capable of enhancing the sharpness of digital images containing memory colors while reducing noise have been developed.
U.S. Pat. No. 6,721,457 discloses a threshold-based adaptive filtering method in which luminance of a pixel window of an input image is determined, the result of the determination is compared with a predefined threshold, and an appropriate adaptive filter is chosen for the input image according to the results of the comparison. In this threshold-based adaptive filtering method, however, only luminance is taken into consideration when determining a filter coefficient, and thus, the filter coefficient may not properly reflect the properties of colors of the input image. Also, this threshold-based adaptive filtering method is not suitable for representing natural colors such as skin tones, sea blue, and green grass, which have different color properties from one another.
U.S. Pat. No. 5,363,209 discloses a sharpness adjustment procedure in which an input image signal is converted into a plurality of luminance-chrominance signals, the sharpness of the input image is determined based on the luminance-chrominance signals, and a filter coefficient for the input image is calculated using a filter function. This sharpness enhancement method can generally enhance the sharpness of images, but is likely to increase graininess and noise when applied to skin tone domains of images.
There are a considerable number of related art image quality enhancement techniques other than those set forth herein. However, none of the related art image quality enhancement techniques can adaptively generate a filter coefficient according to the properties of memory colors such as skin tones, sky blue, green grass, and sea blue.